The present invention relates to a sheet feeder having a switch back path for reversing a feed direction of a sheet.
For an image forming apparatus for performing double-sided image formation processing to a sheet, a sheet feeder having a switch back path for reversing a first surface and a second surface of the sheet by reversing the front and rear of the feed direction of the sheet is used. The sheet feeder feeds in a sheet with an image formed on the first side thereof in an image forming portion to a switch back path, and then feeds out the sheet from the switch back path in a direction opposite to a direction in which the sheet is fed in. The sheet fed out of the switch back path has the first surface and the second surface thereof reversed so that an image is formed on the second surface, and then is fed to the image forming portion.
When the double-sided image formation processing to a plurality of sheets is performed successively, performing the double-sided image formation processing to all sheets requires a long time in a case where a succeeding sheet is fed into a switch back path after a preceding sheet is fed out of the switch back path.
In view of the foregoing, some conventional sheet feeders, as described by Japanese Patent Laid-Open publication No. 05-97305 for example, are configured to feed a sheet being fed out and a succeeding sheet being fed in each other in opposite directions in a switch back path.
The switch back path is connected to a part of the sheet feed path through a branch portion, a sheet is fed into the switch back path via the branch portion from the upstream side of the branch portion in the sheet feed path, and the sheet is fed out of the switch back path to the downstream side of the branch portion in the sheet feed path via the branch portion. At the branch portion, a pair of upstream side rollers and a pair of downstream side rollers that share a driving roller is disposed. The switch back path is provided with a pair of reversing rollers including a reversing driving roller capable of freely rotating both in a forward direction and in a backward direction and a reversing driven roller capable of freely contacting and separating from the reversing driving roller.
In a state where the reversing driven roller is in contact with the reversing driving roller being forward rotating in a feed-in direction, a sheet is fed into the switch back path via the pair of upstream side rollers from the sheet feed path, and fed out by backward rotating the reversing driving roller in a feed-out direction after the rear end of the sheet has passed the branch portion. When the leading edge of the sheet being fed out of the switch back path has reached the pair of downstream side rollers, the reversing driven roller is separated from the reversing driving roller. During a period from a time when the rear end of the sheet being fed out has passed the pair of reversing rollers until a time when the rear end of the sheet being fed in reaches the pair of upstream side rollers, the reversing driving roller is forward rotated in the feed-in direction, and the reversing driven roller is made to be in contact with the reversing driving roller.
The conventional sheet feeders separate the reversing driven roller from the reversing driving roller every time when the leading edge of a sheet being fed out reaches the pair of downstream side rollers, and makes the reversing driven roller to be in contact with the reversing driving roller every time when the rear end of a sheet being fed in reaches the pair of upstream side rollers. In successively performing the double-sided image forming processing to a plurality of sheets, every time when a sheet is fed in and fed out of the switch back path, it is necessary to make the reversing driven roller repeatedly contact and separate from the reversing driven roller so that control of a contact/separation mechanism of the reversing driven roller becomes complicated.
It is an object of the present invention to provide a sheet feeder capable of eliminating a contact/separation operation of a reversing driven roller to a reversing driving roller in successively performing double-sided image forming processing to a plurality of sheets, and capable of simplifying control of the contact/separation mechanism of the reversing driven roller.